Polysulfides have been found useful for many purposes, such as additives or elastomers, as antioxidants for lubricating oils, and useful in the compounding of extreme pressure lubricants.
Also, polysulfides provide a relatively high sulfur content source for presulfiding of metals on hydrotreating and hydrocracking catalysts that are used in various hydrogen refining processes, as well as distillate fuel and lube oil refining catalysts. Further, polysulfides are used as a sulfur passivation agent of heated metal surfaces in furnaces and separation processes related to hydrocarbon/steam pyrolysis processes to produce olefins and diolefins.
A conventional process for producing a polysulfide compound is to react a mercaptan, such as t-dodecylmercaptan, with elemental sulfur, in the presence of a catalyst. However, the resultant polysulfide contains unreacted mercaptans and residual H.sub.2 S.
Another example of the production of a polysulfide product is the reaction product of isobutylene, sulfur, and H.sub.2 S, with n-butyl amine used as a catalyst. A polysulfide containing about 54 wt % sulfur is the resultant product. Such a polysulfide product has been utilized as a catalyst presulfiding agent. Upon standing, this product generates H.sub.2 S which evolves into the vapor space, in an amount up to 2000 ppm H.sub.2 S, as measured using the draeger tube method, or up to 50-60 ppm, as measured by the HS3-BM method, or up to about 30-50 ppm, when analyzed by the GC-headspace method. Because such a product has been sold as a low-odor alternative to dimethyl disulfide or dimethyl sulfide, any generation of H.sub.2 S is undesirable by the industry. Due to the presence of H.sub.2 S, such a polysulfide has been formulated with 0.4 wt % of Tolad SX9200 (known as HSS-2, available from Baker Petrolite). Although HSS-2 works very well, it contains about 5 wt % nitrogen, which adds about 200 ppm of nitrogen to the resultant product. Also, it is suspected that the nitrogen based H.sub.2 S scavenger can contribute to compatibility problems of the resultant formulation if water contamination is encountered.
The presence of H.sub.2 S also contributes to an unpleasant odor, and presents many environmental and safety hazards. Hydrogen sulfide is highly flammable, toxic when inhaled, and strongly irritates the eyes and other mucous membranes.
Treatments to reduce or remove H.sub.2 S have often been called "sweetening" treatments. The agent that is used to remove or reduce H.sub.2 S levels sometimes is called a "scavenging" agent. The sweetening or scavenging of H.sub.2 S from petroleum or natural gas is an example of where H.sub.2 S level reduction or removal has been performed. Many aqueous substrates also have been treated to reduce or remove H.sub.2 S.
Attempts also have been made to remove unreacted mercaptans and residual H.sub.2 S from a polysulfide product. For example, European Patent Application 0 076 376, published on Apr. 13, 1983, discloses that a newly synthesized polysulfide can be deodorized by treating the crude polysulfide with a metal salt of an inorganic or organic acid at an elevated temperature. However, the process requires a lengthy treatment to obtain a satisfactory result, and use of powdered anhydrous salt is preferred, which is more costly.
It is therefore very desirable to reduce the sulfur content of a polysulfide product utilizing a more attractive scavenger agent and method.
It is therefore one objective of the present invention to develop a process to stabilize a polysulfide compound so the subsequent release and/or generation of H.sub.2 S is minimized and the desirable low odor properties of the polysulfide product(s) is maintained for extended periods of time. This is done by reducing unreacted or residual sulfur-containing compounds contaminating the polysulfide.
In addition, the use of an non-nitrogen containing H.sub.2 S scavenger has the added benefit of lowering the amount of nitrogen added to the polysulfide, and subsequently to the catalyst or steam pyrolysis processes.
These objectives are accomplished with the present invention without adversely impacting the desirable properties of the polysulfide as a catalyst presulfiding or metal passivation agent.
Other advantages and features will become more apparent as the invention is more fully disclosed in the following disclosure and claims.